This invention relates generally to a heat actuated fuse and to an apparatus which incorporates such a fuse for protecting electrical circuitry. The invention is particularly useful for interrupting current flow to thick film circuits and components.
A heat actuated fuse has an important advantage over the more common current actuated fuse. Although both types of fuses ultimately open at a predetermined temperature, a heat actuated fuse can be positioned such that it responds directly to the temperature of a protected component or circuit, and temperature is often a better indicator of imminent failure than current. Examples of known heat actuated fuses are described in Great Britain Patent 2,145,295A and U.S. Pat. Nos. 4,533,896, 5,084,691, and 5,097,247.
Great Britain Patent 2,145,295A discloses a thermal fuse disposed on a substrate nearby a resistor, which is also disposed on the substrate. The fuse includes a pair of electrodes defining a gap between them, a gold fuse link extending across the gap to electrically interconnect the electrodes, and a film of solder overlying the gold fuse link and overlapping at least one of the electrodes. When the temperature of the substrate exceeds the melting point of the solder film, the solder is intended to melt, dissolve the gold fuse link, and then retreat from the gap with the dissolved gold to sever the electrical connection between the electrodes.
This fuse is relatively simple to manufacture, but its reliability depends on thorough dissolution of the gold fuse link by the process of leaching. This is a gradual process which begins after the film of solder melts. The gold fuse link then dissolves and is attracted to tin in the solder. The speed of opening of this fuse is relatively slow, and the solder material is limited to a composition containing tin or whatever material will effectively leach the gold or other metal chosen for the fuse link.
U.S. Pat. No. 4,533,86 discloses a fuse for protecting thick film devices deposited on a substrate. The fuse includes two terminal clocks, each including a hole, which are mounted to the substrate in close proximity to each other and with the holes aligned. An electrically conductive fusible link (e.g., solder) is suspended across a space between the terminal blocks and extends into the holes to complete an electrical circuit. The holes are larger than necessary to accept the fusible link so that, when the fusible link melts because of excessive heat, molten material from the fusible link will be drawn into the holes.
Manufacture of this fuse is a complicated and labor intensive process. Manufacture requires boring of holes in the terminal blocks, coating the inner surfaces of the holes and the end surfaces of the terminal blocks with solder, assembling the blocks and fusible link, and then positioning and soldering the assembled fuse onto electrodes provided on the substrate.
U.S. Pat. No. 5,084,691 discloses a thermal fuse comprising a pair of silver conductive pads and a solderable and/or meltable electrically conductive bar extending between the conductive pads. When a conductive bar is applied between the conductive pads, a solder mass structure is applied to the conductive pads, a solder paste is applied through a stencil, and the solder is then heated to provide a connection to the conductive pads. In alternate cases, a solder preform is applied as the fuse element. In this case, the ends of the solder preform are soldered directly to the conductor pads, without the use of a layer of solder paste on the electrodes.
Again, the reliability of this fuse substantially depends on the dissolution of the conductive pads by the process of leaching of the silver material. This is a gradual process which begins only after the fuse is activated. The silver in the pads dissolves to increase the gap when the fuse is activated. The speed of opening of this fuse, therefore, is relatively slow. Furthermore, both the solder material and conductive pad materials are limited to certain defined compositions.
U.S. Pat. No. 5,097,247 discloses an arrangement of materials which function as a fuse and can be deposited on an electrically insulating substrate, using standard printed circuit fabrication techniques. First and second electrodes are disposed on the substrate, defining a gap therebetween. A layer of first solder paste material, having a melting temperature higher than the actuation temperature of the fuse is disposed on at least one of the electrodes. A conductive link disposed on at least a portion of each electrode and on the substrate extends across the gap to provide an electrical connection between the electrodes. The conductive link is comprised of a second solder paste material which has a melting temperature substantially corresponding to the actuation temperature of the fuse.
The melting temperature of the second solder paste, however, necessitates an inert, nitrogen or otherwise non-oxidizing atmosphere during manufacture. Depending upon the circuit board to which the fuse is attached and how the board is processed, the paste may reflow and inadvertently open the fuse.
Accordingly, there are a number of problems and drawbacks to the present technology. There is a need for a fuse and/or a method of manufacture to meet the requirements described above, as well as others known to those skilled in the art.
It is an object of the invention to provide a heat actuated fuse which opens reliably and quickly, but not at process temperatures.
It is another object of the invention to provide such a fuse which can be manufactured simply to open within a wide range of temperatures.
It is yet another object of the invention top provide apparatus incorporating one or more of such fuses for protecting circuitry including one or more electrical elements.
It can also be an object of the present invention to provide a thermal fuse using a solder paste to tin an underlying metal pattern and/or to control the dimensional coverage of the paste and permit further screen printing.
It can also be an object of this invention to provide a thermal fuse prepared using a solder paste and a solder portion and/or chip using standard and/or modified thick film high bred printing technology.
It can also be an object of the present invention to provide a thermal fuse and/or method of preparing the same, such that circuit density is increased.
It can also be an object of the present invention to provide a thermal fuse and/or method of preparing the same, such that the thermal characteristics of the fuse are improved and the solder contact with the substrate is enhanced.
It can also be an object of the present invention to provide a thermal fuse having a preformed solder portion.
It can also be an object of the present invention to provide a method for preparing a thermal fuse without manual attachment of a solder wire and/or without a fluxing operation.
It can also be an object of the present invention to provide a method for preparing a thermal fuse using a removable solder to bond a second solder portion and/or chip to a substrate.
It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. In this regard, the aforementioned objectsxe2x80x94as well as those aspects and features which followxe2x80x94can be viewed in the alternative with respect to any one aspect of the present invention.
Other objects, features and advantages of the present invention will be apparent from this summary and will be readily apparent to those skilled in the art having knowledge of thermal fuse and thick film technology. Other objects, features, benefits and advantages will be apparent from the above as taken into conjunction with the accompanying examples, figures, tables, data and all reasonable inferences to be drawn therefrom.
In accordance with the invention, a heat actuated fuse for opening at a predetermined temperature comprises an arrangement of materials which can be deposited on an electrically insulating substrate by commonly used printed circuit fabrication techniques. First and second electrodes of electrically conductive material are disposed on the substrate in a spaced relationship to define a gap between the electrodes. A layer of a first solder material is disposed on at least one of the electrodes. A conductive solder portion and/or chip is disposed on at least a portion of each electrode and on the substrate and extends across the gap to provide an electrical connection between the electrodes. The conductive portion/chip comprises a predetermined preformed quantity of a non-paste solder material preferably comprising a flux core solder material, which is in contact with the first solder layers on the electrodes. In alternate applications, a second attachment layer of solder can also be used to attach the conductive solder portion and/or chip to the first layer of solder.
Because different solder materials having a large range of respective melting temperatures are readily available, the fuse can be constructed to open at any temperature within a correspondingly large range. The melting temperature of the conductive portion/chip corresponds to the actuation temperature of the fuse. In some applications, the melting temperature of the first layer and optional second layer of solder material used in constructing the thermal fuse are the same or substantially the same as the actuation temperature of the fuse. In alternate applications, the first and optional second layers of solder can have melting temperatures lower than or greater than the actuation temperature of the fuse.
A quantity of flux-containing material can be disposed in contact with the conductive solder portion/chip to effect a rapid and reliable flow of the conducting portion/chip to the first and second electrodes, when the fuse is heated to its actuation temperature. As noted above, however, the conductive solder portion and/or chip can comprise a flux core material. When material of this type is used, the amount of external flux used can be reduced or the external flux can be eliminated.
In another embodiment of the invention, a resistive device is disposed proximate the fuse for effecting heating of the fuse in response to an electrical current passed through the device. Various advantages can be achieved by utilizing such means, independent of the protected circuit element(s), for heating the fuse. For example, by electrically connecting and physically disposing the resistive device such that it is subjected to the same electrical and environmental conditions as the circuitry protected by the fuse, freedom is achieved in the placement of the circuit element(s). Also, types of circuit elements which cannot, without destruction, produce sufficient heat to melt the fusible portion/chip may be protected.
The non-paste solder portion/chip of this invention precludes the need for an inert, nitrogen or otherwise non-oxidizing atmosphere during manufacture. The higher melting point associated with such a solder material can prevent opening the fuse during reflow of the circuit board associated therewith.